The constant development of high-frequency surgery in recent years has led to procedures for contact coagulation (tumor devitalization), to methods for (underwater) tissue sections and for (underwater) tissue vaporization. High-frequency generators which operate with high continuous outputs and very high pulse outputs and/or long activation times are used to perform these procedures. At the same time, however, requirements on the electromagnetic compatibility (EMC) of the high-frequency generators used are increasing because interference with other electromedical apparatus, for patient monitoring or diagnosis for example, is becoming increasingly unacceptable. As a result, the measures necessary to ensure the inherent interference immunity of such high-frequency generators, which nevertheless achieve the high high-frequency outputs required, can only be implemented with a significant amount of technical development effort.
It is known from the prior art to use circuit arrangements including series and/or parallel resonant circuits for the generation of high-frequency power which are fed by power semiconductors in switched mode. For the output characteristics of the high-frequency generators, alongside the properties of the power supply unit (including any closed-loop control systems), the crucial factors are primarily the design of these resonant circuits and the manner in which they are fed, which may also serve as a control variable for closed-loop control circuits. At the same time, it is known that the resonant frequency and the input resistance (and hence the transformation ratio of such resonant circuits) are essentially dependent on the load resistance. In the circuit arrangements known from the prior art for the implementation of high-frequency generators with resonant circuits, this leads to the resonance splitting at a specific frequency, depending on the operating mode, since the series and parallel resonant circuits interact with each other above a certain load resistance range. However, unfavorably high power supply unit currents would be needed, among other things, in order to still achieve the high power output required. On one hand this has an adverse effect on the high-frequency generator's efficiency and on the other it leads to operation of the high-frequency generator with non-sinusoidal input current as a result of which the EMC characteristics of the high-frequency generator also deteriorate.
FIG. 4a and FIG. 5a illustrate circuit diagrams of different resonant circuits known from the prior art, such as are used for generating high-frequency power outputs. In these cases, it is possible to integrate a (leakage) transformer using the structure Lp, L2 for an additional impedance transformation which is independent of the load resistance.
FIG. 4b, c and FIG. 5b, c illustrate the curves of the resonant frequency fr, the filter input resistance RE and the maximum output Pa,max and output voltage Ua,max associated with each circuit diagram as a function of the load resistance RL. Both of the latter additionally depend on the type of supply.
In this case, a supply with a square-wave voltage of the corresponding resonant frequency and a power supply unit with a maximum output voltage U0 and a maximum output current I0 is assumed. Under these conditions, the optimum load resistance Ropt=U0/I0 transforms into the optimum filter input resistance REopt=8/pi^2*Ropt.
The power supply unit operates in the current limiting for RE<REopt and in the voltage limiting for RE>REopt. In accordance with the properties of ideal resonance circuits, resonance points at which the filter behaves like a series resonant circuit (SRC) or parallel resonant circuit (PRC) are described as series resonances (SR) or parallel resonances (PR). FIG. 4b, c and FIG. 5b, c illustrate series resonances by means of a broken line and parallel resonances by means of a continuous line.
In addition, FIGS. 4b and 5b each illustrate all the resonance points possible for the filter, but only the operating frequencies actually occurring with the type of frequency feedback chosen in each case are represented by means of a continuous line. In FIGS. 4c and 5c, the filter input resistance RE is represented only for these frequencies by means of a continuous line, the maximum output Pa,max by means of a broken line and the maximum peak output voltage Uamaxp by means of a dot-dash line.
The loaded qualities of the individual resonance circuits Q1=1/RL*sqrt(L1/C1), Q2=1/RL*sqrt(L2/C2) and Qp=RL*sqrt(Cp/Lp) are helpful for characterizing the curve shapes.
Branching of the resonant curves takes place at load resistance Ro which emerges in FIG. 4b from Qp=Q2 and in FIG. 5b from Qp=Q1+Q2. As illustrated here, branching then takes place at precisely the point when the resonant frequencies of the individual resonant circuits coincide.
FIG. 4a shows a resonant circuit known from the prior art for generating outputs having a parallel resonant circuit (PRC) at input A, B comprising a capacitor Cp and an inductor Lp, a series resonant circuit having an inductor L2 and capacitor C2 and a load RI, at output C, D. In conjunction with the voltage supply usually used, this arrangement is unsuitable for fulfilling the requirements for high high-frequency output and a good level of efficiency because a highly non-sinusoidal input current appears in the process. Although a current supply would remedy matters appropriately, supplying by using a power source is, however, comparatively complex.
It is apparent from FIG. 4b that the resonant frequency for small load resistances splits because the parallel resonant circuit PRC and the output series resonant circuit SRC interact. As a result, the resonant frequency increases as the load resistance decreases.
FIG. 5a shows a resonant circuit known from the prior art for generating outputs having a series resonant circuit SRC at input A, B comprising an inductor L1 and a capacitor C1, a parallel resonant circuit PRC having a capacitor Cp and an inductor Lp, a series resonant circuit SRC having an inductor L2 and a capacitor C2 and a load RL, at output C, D.
Although this circuit configuration is suitable for a voltage supply, it has the drawback that the series resonance splits for large load resistances and shifts severely as a result as is apparent from FIG. 5b since the input SRC and PRC interact with each other and at the same time the associated input resistances assume such small values that unfavorably large power supply currents would be necessary in order to achieve the desired output. It is necessary to switch over to parallel resonance to prevent this. However, this leads to an operating mode with non-sinusoidal input current which would have a negative impact on the EMC characteristics of the arrangement:
As explained above, the measures known from the prior art for minimizing the drawbacks referred to have therefore consisted so far in providing the circuit arrangement of an high-frequency generator with either a current or a voltage supply depending on the application. However, the circuit engineering required to implement this solution is frequently complex.